1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory and a semiconductor device, and more particularly, to a circuit technique for boosting a power supply voltage.
2. Description of the Related Art
In a flash memory that is one of the non-volatile semiconductor memories, a power supply voltage is boosted to internally generate voltages for programming and erasing. For example, in program verification, a power supply voltage (for example, 3 V) is boosted to generate a high voltage (for example, 6 V), which is applied to the gate of a non-volatile semiconductor memory cell (word line) and the gate of a gate transistor for selecting the non-volatile semiconductor memory cell. In programming following the program verification, the power supply voltage is boosted to generate a higher voltage (for example, 9 V), which is applied to the gates of the memory cell and the gate transistor. The program verification is a read operation for verifying that the memory cell has a given threshold voltage. If the verification fails (the memory cell does not have the given threshold voltage), the cell is further programmed to raise the threshold voltage. In this manner, the program verification and programming are repeatedly carried out in turn until the memory cell has the given threshold voltage.
A charge pump circuit implements the boosting. The above-mentioned example employs a charge pump circuit that generates a voltage of 6 V from the power supply voltage, and another charge pump circuit that generates a voltage of 9 V from the power supply voltage. If the charge pump circuit operates slowly (low drivability), it will take a long time to complete programming and erasing. The boosting speed can be improved by increasing the size of a capacitor, whereas this needs a large circuit area and a large power is consumed. Generally, the most importance is attached to the circuit area, and the boosting speed is sacrificed to a certain extent.